feat(cells): make email capture demo mode control-silo only

[lynnagara]

note: it is safe to ship frontend + backend together in this case as backend currently works in both silos

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #104488      +/-   ##
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+ Coverage   80.53%    80.54%   +0.01%     
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  Files        9350      9350              
  Lines      400099    400330     +231     
  Branches    25660     25660              
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+ Hits       322216    322446     +230     
- Misses      77415     77416       +1     
  Partials      468       468              

[kcons]

For my education, why is this safe? It'd seem that this restricts the endpoint to control and thus old UI code would be potentially broken until a new UI push, so we'd want to make sure the UI change goes out before the backend. Do we not limit by silo in the API this way? Or is "ui deploys first" a safe enough bet that it's not worth worrying about?